You do not know that wavelength = 4L.
The wavelength can be 2(L/1), 2(L/2), 2(L/3), 2(L/4), 2(L/5)....etc
You also know that v = f x wavelength. And v does not change
The values you have been given should enable you to write down 2 simultaneous equations from which you can get the answer.